This invention relates in general to a cryogenic separation system, and in particular to a process and apparatus for obtaining gaseous oxygen and gaseous nitrogen by the low-temperature rectification of air in a double rectification column, wherein a process stream is warmed in the cold section of a reversible heat exchange unit against entering air and is thereafter engine-expanded.
Processes for the separation of air by means of low-temperature rectification are known wherein the raw air is cooled, in reversible heat exchangers, such as, for example, regenerators or "Revex", against gaseous separation products, freed of water vapor and carbon dioxide, and fed, after partial liquefaction, into the high-pressure column of a double rectification column. An air fraction withdrawn from the high-pressure column is warmed in the cold section of the heat exchangers and, after engine expansion, introduced into the low-pressure column of the double column.
Thus, according to the conventional practice, the removal of the water vapor and carbon dioxide from the raw air requires the recycling and/or warming of a process stream (e.g., an air fraction from the high-pressure column) in the cold section of the heat exchangers. To obtain a complete purification of the air, this process stream (also called the compensating stream) must amount to about 11-13% of the amount of the total air throughput. Deviations from this range result in unstable reversing ratios and finally in carbon dioxide accumulations in the liquid oxygen pool in the condenser-evaporator of the low-pressure column. Carbon dioxide obstructions diminish the heat exchange efficiency and promote the formation of sites of explosion in the condenser-evaporators due to local enrichment of hydrocarbons on account of the dry evaporation of the oxygen in the evaporator passages obstructed by carbon dioxide.
During normal operation of the evaporator-condensor with passages not obstructed by carbon dioxide an internal liquid oxygen circulation is formed, the oxygen stream traversing the passages in upward direction. The rate of evaporation from the circulating liquid oxgyen usually amounts to no more than about 20 to 40%. Hydrocarbons contained in the liquid oxygen remain in the liquid phase. Carbon dioxide obstructions diminish the flow cross section of the passages, thereby increasing the resistance of flow and reducing the liquid oxygen circulation to an amount where all liquid introduced is evaporated (dry evaporation), the hydrocarbons contained in the liquid oxygen being deposited during evaporation on the inner walls of the passages.
The compensating stream is customarily engine-expanded in a turbine after giving off its cold to the entering raw air. The thus-obtained refrigeration serves for covering all refrigeration losses of the process. In air separation plants of certain sizes where all separation products are produced in the gaseous phase at ambient temperature, the expansion of the compensating stream generates a significantly larger quantity of cold than actually required by the process. This excess becomes greater with increased plant size, as the larger the plant, the smaller the specific insulating losses. For example, whereas about 20-25% of the employed air must be expanded in the turbine to cover the refrigeration requirement in smaller plants, the expansion of no more than 7% in most cases is sufficient in modern large-scale plants.
Since on the one hand, the compensating stream must not drop below 11-13% of the air throughout but, on the other hand, an expansion of 7% of the employed air is entirely sufficient, excess cold is produced by the engine expansion of the compensating stream. Thus, additional energy must be expended to convert the liquid oxygen, externally of the process, from the liquid phase into a gaseous phase at ambient temperature. In other words, energy is required to remove the excess cold. In order to save this additional vaporizing energy, the compensating stream is, under practical conditions, engine-expanded in the turbine, but only after the inlet pressure is first lowered to such an extent that the remaining pressure expansion in the turbine yields precisely the required amount of cold. However, such a mode of operation is still extremely unsatisfactory due to the high thermodynamic energy losses incurred thereby.